The dienophilic reactivity of allenic trichloromethyl sulfoxides under ultrasound irradiation has been investigated. A considerable rate enhancement has been observed for cycloaddition of the unsubstituted and γ-monosubstituted allenyl trichloromethyl sulfoxides to cyclopentadiene. With the corresponding γ, γ-disubstituted allenic sulfoxides, a competing 1, 2-to 1, 3-diene isomerization is observed.
